Abstract: Based on analyzing the characteristics of balanced flowing path, and taking ideal filling pattern as goal, through introducing geometric boundary data of the injection part as basic information, a new rule of optimal positioning injection gate is put forward. Meanwhile, it takes the flat-shaped part as sample, puts forward a series of mathematical models for the gate position optimization as well as its iteration strategy. Generation and manipulation of the key points set that represents the geometric boundary of the part are provided as well. Based on these mathematical models and iteration method, the software for calculating the optimal gate position was developed accordingly, giving satisfying results with practical cases.